Roughly 100 firefighters are on hand. At this point, there's no indication of what caused the fire. Via MyNorthwest.com:

"We are attacking this, at this point, as a defensive operation from the outside of the building. We have firefighters surrounding the whole building with hand lines and deck lines. We're just getting as much water on the fire as possible," said Vander-Houwen [with the Seattle Fire Department].

Airport Way South is closed, traffic is backed up both ways on I-5.